The hydrocracking of heavy petroleum fractions is of great importance in current refining systems, since it makes it possible to obtain a wide variety of light products such as aviation fuel, medium distillates and light fuel oils from heavier feeds of lower intrinsic value. One advantage of hydrocracking over other conversion processes, such as catalytic cracking, is that it yields medium distillates of excellent quality, whereas the gasoline from hydrocracking usually has a lower octane number than that obtained by catalytic cracking. Furthermore, the great flexibility of hydrocracking means that the production of different fractions can be adapted to market demand.
Conventional hydrocracking catalysts are bifunctional, i.e. they consist of the combination of a hydrogenating function and an acid function. The hydrogenating function arises from the presence in the catalyst of one or more metals in Group VIB of the Periodic Table of the Elements, such as molybdenum or tungsten, or from a combination of one or more Group VIII metals (preferably non-noble metals) such as nickel, cobalt or iron with Group VIB metals. The acid function is generally associated with a porous support with a high specific surface area having surface acidity, such as halogenated alumina, mixed oxides such as amorphous silica-alumina, or zeolites.
Both the selectivity for various products and the activity of a bifunctional hydrocracking catalyst are largely determined by the balance between the hydrogenating function and the acid function. When the acid function is weak and the hydrogenating function is strong, the catalyst is characterized by having low hydrocracking activity, which makes it necessary to work at high (above 400° C.) reaction temperatures or at very low (generally lower than 2 h−1) space velocities (volume of feed to be treated per unit volume of catalyst and per hour), and high selectivity for medium distillates. By contrast, when the acid function is strong and the hydrogenating function is weak, the catalyst is characterized by having high hydrocracking activity but low selectivity for medium distillates. A good hydrocracking catalyst must therefore have an appropriate balance between the acid function and the hydrogenating function.
Supports having a low acid function that are currently most used in the formulation of conventional hydrocracking catalysts include, most notably, amorphous silica-alumina. Hydrocracking catalysts based on amorphous silica-alumina have good selectivity for medium distillates but, as stated above, they are characterized by having low activity.
Supports having a stronger acid function include zeolites. Zeolites, specifically zeolite Y with a faujasite structure, are involved in the formulation of new-generation hydrocracking catalysts. Thus, hydrocracking catalysts based on zeolite Y have greater activity than conventional catalysts based on amorphous silica-alumina, although their selectivity for medium distillates is generally lower than that of the latter. Some hydrocracking processes that use zeolite-Y-based catalysts are described, for example, in patents U.S. Pat. No. 3,269,934 and U.S. Pat. No. 3,524,809.
The activity and selectivity of a zeolite-Y-based hydrocracking catalyst can be altered by modifying the acidity of the zeolite, which depends largely on its chemical composition, and more specifically on the ratio between the silicon atoms and aluminum atoms (Si/Al ratio) that determine its crystal structure. It is well known that the presence of an aluminum atom in tetrahedral coordination within the crystalline network of the zeolite generates charge deficiency that is compensated for by a proton, thus giving rise to the formation of a Brönsted acid center. It is therefore possible, in principle, to control the acidity of the zeolite by varying the Si/Al ratio within the network.
Zeolite-Y-based hydrocracking catalysts with a high Al content in the network (low Si/Al ratio) have high activity since they have a greater concentration of Brönsted acid 1 centers. However, these catalysts have low selectivity for medium distillates since the presence of a high number of acid centers promotes secondary cracking reactions promoting the formation of lighter products, such as gases and naphtha. On the other hand, hydrocracking catalysts containing zeolite Y with a low Al content in the network (high Si/Al ratio), and therefore a low acid center concentration, are more selective for medium distillates, although they have lower hydrocracking activity. In order to reduce the Al concentration in the network and to achieve the appropriate range of Si/Al ratios in hydrocracking catalysts, the zeolite Y must undergo post-synthesis treatments to remove aluminum, since this zeolite cannot be synthesized with a high Si/Al ratio. Such aluminum-removing treatments generally require severe hydrothermal conditions that result in a partial loss of zeolite crystallinity.
It would therefore be highly desirable to have a hydrocracking catalyst with good activity and selectivity for medium distillates, based on a microporous crystalline solid with a topology such that it has cavities with a high volume similar to that of large-pore zeolites, like zeolite Y, and which can be obtained with a high Si/Al ratio in a single synthesis step, avoiding subsequent aluminum-removing processes.
The microporous crystalline solid known as ITQ-21 is described in Spanish patent application P200101145, which corresponds to PCT Publication No.WO02/092511, that entered U.S. national chase on Nov. 14, 2003 as U.S. Ser. No. 10/714,571, and issued as U.S. Pat. No. 6,849,248 on Feb. 1, 2005. However, its use as a hydrocracking catalyst component is neither suggested nor described in said application. Spanish application P20012287, which corresponds to PCT Publication No. WO03/029387, that entered U.S. national phase on Apr. 2. 2003 as U.S. Ser. No. 10/817,772, and issued as U.S. Pat. No. 6,998,037 on Feb. 14. 2006, also relates to said crystalline solid material, specifically to its use in cracking. However, this application does not describe its use as a hydrocracking catalyst component in conjunction with Group VIB or VIII metals.